The present invention relates to a system for assessing DSL capability of telephone lines, and more particularly, a DSL test system that employs test signals in both narrow band frequency ranges and mid band frequency ranges along a selected test path of a telephone line to identify line characteristics and line conditions that could affect DSL service on the telephone line.
With the explosion in the growth of Internet usage among both businesses and households, telephone companies have been pressured to provide affordable, high bandwidth access that will support high-speed multimedia services, such as video on demand, high speed Internet access, and video conferencing. To meet this demand, telephone companies are increasingly turning to digital subscriber line (DSL) technology. As used herein, DSL is a generic term for a group of higher-rate digital subscriber line communication schemes capable of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. DSL has been further separated into several different categories of technologies according to specific expected data transfer rates, the types and lengths of the medium over which data are communicated, and schemes for encoding and decoding the communicated data. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), and VDSL (Very high data rate Digital Subscriber Line). DSL technology, while having several different embodiments, can provide throughput rates over 100 times faster than that available through traditional 56 Kb/s modems. As with most telecommunication products and services, DSL service providers need reliable, cost effective means of testing or qualifying the DSL capabilities of the basic telephone lines as well as providing periodic or on-demand assurance of DSL service to its subscribers.
One way of addressing the problem of DSL service assurance is for a service provider to leverage the capabilities of their existing testing infrastructure, such as the common Plain Old Telephony System (POTS) test hardware. As part of the POTS test environment, many service providers today use an automated loop test system (LTS), for analyzing reported troubles on subscribers' telephone lines. An LTS selectively connects to the central office terminals of twisted pair telephone wiring and conducts electrical tests on metallic circuits. Such a system can apply AC voltage across a wire pair, between the Tip (T) wire and ground, and between the Ring (R) wire and ground, and take appropriate measurements to determine characteristic impedances. The LTS can also measure the DC resistance between the wires and between each wire and ground. The LTS stores a list of DC and AC resistance/impedance values that correspond to certain line conditions, e.g. shorts, opens, normal telephone set connections, etc. The LTS makes decisions as to the presence or absence of different types of faults by comparing the test result values to its stored list of fault threshold values. However, LTS tests provide limited information regarding the transfer characteristic of the loop or telephone line, particularly with respect to the frequency ranges affecting DSL services. Deployed line test systems cannot accurately determine if a line is ‘Test OK’ for DSL service and can give erroneous test results based on voice band measurements and POTS dispatch and fault analysis. These erroneous test results include incorrectly stating ‘Test OK’ when loop or telephone line is faulty or inadequate for DSL service, and incorrectly identifying loop failures associated with voice band that do not impair DSL service.
Another common approach for DSL qualifications or service assurance is for a service provider to use their network management infrastructure to access status information about each subscriber's DSL service from the DSLAM. A DSLAM typically makes available the following information about the status of each subscriber's data connection (ATU-C and ATU-R): serial number, version number, current signal-to-noise margin, current attenuation, current operating status (including loss of framing, loss of power, loss of signal quality), current output power, current attainable rate, current transmit rate, current receive rate.
Unfortunately, the DSLAM does not support direct measurements of the switching fabric, central office splitter, the central office wiring, the outside plant wiring, and premises wiring. Thus, while the DSLAM information can indicate that a service is unavailable or impaired it cannot perform necessary analysis to determine root cause of a fault, type of fault, or location of a fault, except where the fault is within the DSLAM itself
Yet another approach is to introduce a DSL test unit into the path between the ATU-C and ATU-R using a Test Access Matrix (TAM) just beyond the CO splitter. The TAM provides a test path that circumvents the splitter allowing high frequency based measurements to be performed on the line. These measurements provide a direct indication of ingress noise and other line failure modes. In addition to the additional test hardware required, a critical limitation to this approach is that establishing a test path through the high frequency measurement unit disrupts the subscriber's DSL service for the duration of testing, which could last anywhere from thirty seconds to over three minutes or more.
The presently disclosed DSL test system and methods overcome one or more of the problems identified above.